Well here's one. When shading, it is best to choose a light source. Your pic, for instance, looks like it's coming from the right. So start from there. Draw in your sun and decide on what to shade in darker than others. You want to shade in all that the light is falling on. Choose simple directions to begin with, like right, left, up and down, then work from there on out, like central lighting and so forth. Does that help any, or is it too confusing?

CS put it in a really good explaination with words. Better than me actually. And here's a pic I did that can help with visual aids. My pictures light source is coming from the left, hence forth; this is what the shading looks like.

(Only one I could pick out of my gallery with decent shading and what not. xP)